Existing centralized power generation systems are not able to provide a sustainable solution for the growing energy demands. Centralized power generation systems are very costly, require maintenance and lack reliability. They also significantly contribute to environmental pollution by having harmful emissions. Renewable energy in the form of decentralized power generation systems is a very promising solution to the world energy crisis. Soon localized renewable energy power generation systems will be responsible for providing energy to the end-users.
Photovoltaic (PV) power generation systems offer a very clean and practical energy source. PV power conditioning systems extract the power from the PV panel and deliver the power to the utility grid and local loads. The main challenges to the more widespread acceptance of PV power conditioning systems are efficiency, reliability and robustness. PV power systems should be able to efficiently deliver power from the PV panel to the grid and local loads under different operating conditions.
A PV power conditioning system, usually, consists of two stages. The first stage is a DC/DC converter. The DC/DC converter is responsible for boosting the voltage at the output of the PV panel and for providing galvanic isolation as required by regulatory standards. This stage is also responsible for extracting the maximum power from the PV panel and for delivering this power to an intermediate DC-link. The second stage is usually a conventional DC/AC inverter. The DC/AC converter converts the DC power to AC power which is deliverable to the utility grid.
One of the main challenges of the first stage is that of processing the power very efficiently. Thus, the efficiency of the DC/DC converter is of great importance. Another main challenge is that of quickly tracking the maximum power produced by the PV panel. A further main challenge is that of reliability. The DC/DC converter should operate very robustly for a wide range of operational conditions due to erratic weather.
Switching losses greatly contribute to the overall losses of the DC/DC converter. Soft-switching techniques are able to significantly attenuate switching losses. Therefore, soft-switching is necessary to achieve good efficiency in DC/DC converters. Soft-switching is realized when either the voltage across the power semiconductor or the current flowing through the power semiconductor is zero during the switching transitions. Because of this, zero voltage switching (ZVS) and zero current switching (ZCS) are among the soft-switching techniques used to improve the efficiency of the DC/DC converter. In conventional full-bridge PWM (pulse width modulation) converters, ZVS is achieved by using the energy stored in the leakage inductance to charge and discharge the output capacitances of the MOSFETs in the circuit. Because of this, the range of ZVS operation is highly dependent on the load and on the transformer leakage inductance. This dependence is one of the main limitations of the conventional full-bridge converter. Conventional full-bridge converters are not able to ensure ZVS operation for a wide range of load variations as the range of ZVS operation is dependent on the load and the transformer leakage inductance.
Another difficulty related to the conventional full-bridge phase-shift DC/DC converter is the performance of the output diodes. The interaction between the leakage inductance of the transformer in the converter and the output filter in the converter significantly degrades the performance of the output diodes. This interaction causes very high voltage spikes across the output diodes as well as lossy commutations of the output diodes. These lead to duty cycle loss and significant oscillatory current in the primary side of the transformer. These problems are intensified when the switching frequency of the converter increases. Because of these issues, the conventional full-bridge phase-shift DC/DC converter topology does provide good efficiency and performance for PV power conditioning systems. That being said, there are a number of references which propose solutions for improving the performance of the output diodes. While some of these proposed techniques mitigate the issues pertaining to the output diodes in conventional full-bridge converters, these solutions require extra active and passive components. These requirements significantly offset whatever advantages these solutions may offer.
Resonant converters are able to provide soft-switching while also achieving good efficiency. The operational range for resonant converter should, however, be extended. Generally, in order to extend the range of soft-switching in resonant converters, the resonant circuit should be designed such that there is enough circulating current in the resonant circuit. This, however, compromises the performance of the resonant converter and results in a lower efficiency due to extra losses caused by the circulating current. In addition to this issue, a significant amount of current ripple passes through the resonant components in this application. Because of these problems, resonant circuits are usually bulky and, as such, any power conditioning system using resonant converters will have its power density compromised.
Because of the above shortcomings of the prior art, there is a need for topologies and circuits which can be used in the first-stage DC/DC converter as a part of a two-stage power conditioning system.